1. Field of the Invention
This invention relates generally to an optical fiber crack detector and, more particularly, to an optical fiber crack detector that employs fiber Bragg grating (FBG) sensors.
2. Discussion of the Related Art
Many devices, machines and associated systems employ moveable components that may interact with each other in a manner that may cause undesirable wear, defects, cracks, etc. on the components. For example, turbines, compressors and other machines include motors that rotate shafts having blades and other elements disposed thereon. Operation of such machines may cause unwanted contact of the blades and other moving components with housings and other structures within the machine. This unwanted contact could be caused by many factors, such as thermal expansion, high shaft rotation speed, motor surge, etc. Also, some of these types of machines may employ what are known in the art as knife-edge seals that include a thin edge positioned on a rotating member that contacts a stationary structure and provides a pressure seal from one side of the edge to the other. That is, the knife edge seal limits flow and results in a differential pressure. This rotating contact point between the knife-edge and the structure must be small to limit flow, thus causing wear on both components if they inadvertently touch during machine operation, typically due to non-standard operating conditions, such as surge.
Such component wear may be excessive enough where it would affect the performance and operation of the component, system or machine that they are a part of. Various things can be done to reduce the wear, such as providing lubricants, appropriate low friction materials, favorable orientation between components, etc. However, these available remedies are not always adequate, and excessive wear may still occur. Some machines and systems allow component wear to be detected by visual inspection. However, there are many applications where such visual inspection is not possible, or is not feasible because the time, cost, labor, etc. necessary for the inspection is too great.
Many attempts have been made in the art to detect component wear and other defects by providing sensors within the component at the wear location that detect the removal of material and wear on the component. For example, it is known to embed optical fibers into the surface of a component at a location where it is desired to detect wear, and use a light beam propagating down the fiber and suitable detection circuitry to determine if the fiber has been broken as a result of the wear. Other systems that employ optical fibers are also known.
U.S. patent application Ser. No. 12/724,531, filed Mar. 16, 2010, titled Fiber Optic Sensor System for Detecting Surface Wear, assigned to the assignee of this application and herein incorporated by reference, discloses a fiber optic sensor for detecting surface wear. One or more fibers are provided within the component being detected, where one end of the fiber including a re-emission portion is positioned near the wear surface. A light beam propagating down the optical fiber contacts the re-emission portion, creating a return beam that is detectable. The re-emission portion can be a reflective element that reflects the beam or a florescent element that fluoresces in response to the beam. If the wear of the surface goes deep enough into the component where the re-emission portion is worn away or otherwise significantly damaged, then the return signal is not provided from the re-emission portion, which indicates that the wear of the component has reached a certain depth. However, this type of wear detection sensor has limitations. For example, it is typically necessary to provide the re-emission portion at the end of the fiber.
The detection of wear on various components in the types of machines discussed above extends to the detection of actual cracks that may form in those components that occur as a result of machine use. Crack detection gages are known in the art that include equally spaced parallel strips of a conducting material that terminate to a common conductor at each end. The strips are typically attached to the component being detected by mechanical bonding, such as by cements or epoxies. Each of the individual parallel conductive strips is known to fail at approximately 2% strain. By monitoring the change in electrical resistance as a result of a failing strip, the number of broken conductive strips can be deduced, and the length of the crack can be indirectly determined.
This method includes a number of drawbacks including that the crack gage indirectly measures crack growth, element fabrication variations increase with measurement uncertainty, and the gages are limited in size due to the nature of the element, where the resistance goes up with length. Also, because the gage is electrical based it is susceptible to noise from electromagnetic fields common in certain machinery, and the gage requires a pre-determined location to be known.
It is well known that a fiber propagation core diameter of 10 μm provides multi-mode propagation. Because a 10 μm diameter cable is susceptible to breakage due to strain, it has been proposed that it be installed in a parallel grid pattern that runs perpendicular to the expected crack formation direction. It has previously been proposed to provide an array of these optical fibers on or in a substrate that is to be monitored for crack growth.
U.S. patent application Ser. No. 12/945,957, titled, Sensor Apparatus for Detecting and Monitoring a Crack Propagating Through a Structure, filed Nov. 15, 2010, assigned to the assignee of this application, discloses a crack detection system of this type. The '957 application discloses a sensor apparatus for detecting and monitoring cracks that includes a plurality of parallel optical fibers mounted to a structure being detected. A distal end of each optical fiber is coated with a fluorescent material and a light beam propagating down each optical fiber causes the fluorescent material to fluoresce, which generates a return signal. If one or more of the optical fibers is severed as a result of formation of a crack, that fiber will not provide a return signal, which provides an indication that a crack has formed, where the number of severed fibers provides an indication of the length of the crack.